1. Field of the Invention
This invention relates to an improved method for the thermal dewatering of solid carbonaceous substances and, more particularly, to a method for the thermal upgrading by dewatering of young coals in an efficient and economic manner. The term "young" coal as used herein describes a class of coals falling outside the range of hard coals and includes subbituminous coal, lignite and unconsolidated brown coals. Other carbonaceous substances suitable for use include peat, wood, vegetable material, sewage, sludge and the like.
It is highly desirable that such carbonaceous materials be thermally upgraded in an efficient and economic manner so as to improve their heat generating capabilities, thereby becoming useful in applications involving thermal decomposition and gasification processes. Furthermore, such an upgrading for a pipeline slurry also results in a considerable reduction in transportation costs.
In the United States vast deposits of various grades of young coal reside, particularly in the western half of the nation. These deposits represent a potential solution to the modern energy crisis and resulting fuel shortages. Unfortunately, young coal as mined usually contains a substantial amount of moisture, and it is essential that at least the majority of this water be removed from the coal in order to render it suitable as a fuel.
It is known that at high temperatures coal not only will lose this chemically attached water, but also will undergo a change in structure so that no substantial reabsorption of the water will occur even if the coal is kept in a water phase under high pressure. Such resistance is due to a chemical change in the coal itself, a phenomena known as coalification.
2. Description of the Prior Art
A number of attempts have been made to develop an economic and efficient process of dewatering young coal and other similar carbonaceous substances by subjecting the material to a variety of heat treatments at elevated pressures. A common theme underlying these efforts is that in order to convert young coal to a lesser water bearing substance there should be as little water as possible in the immediate environment, particularly during depressurization and cooling, so that a minimum of water is required to be separated from the coal after the heat treatment.
Several processes have heretofore been used or proposed for treating young coal so as to render it more effective as a solid fuel. These processes usually involve a partial drying of the coal in its as-mined condition to reduce its moisture content, followed by a subsequent processing to render the coal more impervious to moisture. Another class of processes has involved dehydrating the coal initially to a low moisture content by conventional drying methods; however, the amount of heat required to energize the drying gas stream to a temperature required for the dehydration of young coal can be as high as 25% in terms of the heat value of the coal based on the amount of young coal that is originally treated.
U.S. Pat. No. 3,552,031, Evans et al, discloses the separation of contained water from a stream of solid, non-slurried brown coal and other organic materials by treatment of the organic materials in the presence of a fluid medium at an elevated temperature of about 250.degree. C. and at pressures exceeding the saturation pressure.
U.S. Pat. No. 4,052,168, Koppelman, discloses a process for upgrading lignitic type coals by treating the moist, mined solid coal to an autoclaving process at very high temperatures (500.degree.-600.degree. C.) and elevated pressures for a time sufficient to convert the moisture and a substantial portion of the volatile organic constituents therein to a gaseous phase, consequently producing a controlled thermal restructuring of the chemical structure.
U.S. Pat. No. 3,922,784, Verschuur et al, discloses a process for the upgrading of carbonaceous substances, particularly brown coal, which involves a heat treatment of a slurried coal stream at a temperature of at least 150.degree. C. and a pressure above the vapor pressure of water at the corresponding temperature. The slurry is pressurized at a temperature below 100.degree. C. prior to the heat treatment.
Canadian Pat. No. 1,020,477, Wasp, discloses a process for increasing the heat and coal concentrations of a solid, carbonaceous slurry by thermal dewatering the slurry at elevated temperatures and pressures; the treated slurry is then more suitable for pipeline transportation to a designated point.
However, the processes of the prior art which are directed at dewatering a coal slurry either fail to remove a sufficient amount of water from the coal particles or require too large an amount of energy to be economical.
The process of the present invention overcomes the problems and disadvantages of the prior art methods through an improved thermal dewatering process leading to the production of an upgraded solid heating fuel in a novel, economic and energy efficient manner.
An object of this invention is to provide an improved method for the effective and economic dewatering of young coal and other moisture bearing carbonaceous slurried substances in order to use the resultant product as an improved feed coal in a variety of energy requiring processes.
It is another object of this invention to economically separate as much of the surface and chemically attached water from the coal as possible, without permitting the water to be reabsorbed into the coal.
It is still another object of the invention to utilize and recover the heat and pressure generated by the various unit operations of the process in an efficient manner.